The present invention relates to methods of making absorbent cellulosic sheet in general, and more specifically to a wet crepe process wherein a web is dewatered, thereafter creped and dried with an impinging gaseous stream on a rotating cylinder.
Wet crepe processes for making absorbent sheet, such as tissue and towel products, are known in the art. There is disclosed, for example, in U.S. Pat. No. 5,851,353 to Fiscus et al. a method for can drying wet webs for tissue products wherein a partially dewatered wet web is restrained between a pair of molding fabrics. The restrained wet web is processed over a plurality of can dryers, for example, from a consistency of about 40 percent to a consistency of at least about 70 percent. The sheet molding fabrics protect the web from direct contact with the can dryers and impart an impression on the web. Due to low heat transfer coefficients, can drying after a crepe operation can be expensive both in terms of operating costs and capital investment.
Other wet crepe processes, specifically wet crepe, through air dry processes have been suggested in the art and practiced commercially. One such process is described in U.S. Pat. No. 3,432,936 to Cole et al. The process disclosed in the ""936 patent includes: forming a nascent web on a forming fabric; wet pressing the web; drying the web on a Yankee dryer; creping the web off of the Yankee dryer; and through air drying the product. Another wet crepe, through air dry process is suggested in U.S. Pat. No. 4,356,059 to Hostetler. In the ""059 patent there is disclosed a process including: forming a nascent web on a forming fabric; drying the web on a can dryer; creping the web off of the can dryer; through air drying the web; applying the dry web to another Yankee dryer; creping the web from the Yankee dryer and calendaring the product.
Wet crepe, throughair dry processes have not met with substantial commercial success since the process rates, product quality and machine productivity simply could not meet the demanding cost/performance criteria required in the industry. Through dry processes generally require high permeability webs and are difficult to practice on a web that has been compactively dewatered or formed with a substantial portion of secondary (recycle) fiber.
There is provided in accordance with the present invention a method of making absorbent sheet by way of a wet-crepe process wherein the wet-creped web is after-dried with an impinging stream of heated air or a heated gas stream. Unlike through-dry processes, after drying with impinging air can be accomplished on a web with relatively low permeability if so desired and is thus suitable in processes for making paper tissue and towel products where a large proportion of secondary fiber is employed or the web is mechanically compressed. Inasmuch as the drying medium need not flow through the web, greater manufacturing flexibility is thus achieved on a single production line.
There is provided in one aspect of the present invention a method of making absorbent sheet from cellulosic fiber comprising the steps of: (a) depositing an aqueous cellulosic furnish on a foraminous support; (b) dewatering the furnish to form a cellulosic web; (c) applying the dewatered web to a heated rotating cylinder and drying the web to a consistency of greater than about 40 percent and less than about 80 percent; (d) creping the web from said heated cylinder at said consistency of greater than about 40 percent and less than about 80 percent, and optionally wet shaping the web such that the web is rendered suitable for impingement-air drying; and (d) drying said web with an impinging heated gaseous medium subsequent to creping the web from the heated cylinder to form said absorbent sheet.
The web is preferably dewatered to a consistency of at least about 30 percent prior to being applied to the heated cylinder, and more preferably, the web is dewatered to a consistency of at least about 40 percent prior to being applied to the heated cylinder. On the cylinder, the web is typically dried to a consistency of at least about 50 percent prior to being creped, and in many cases the web is dried to a consistency of at least about 60 percent prior to being creped. In some embodiments, the web is dried to a consistency of at least about 70 percent prior to being creped.
In preferred embodiments, the web is relatively highly bulked immediately after wet creping such that the web is open and can be efficiently dried with an impinging gaseous medium. Thus, the characteristic void volume (hereinafter defined) of the web immediately after creping is at least about 6 gms/gm, more preferably at least about 7 gms/gm and in some preferred embodiments exhibiting a characteristic void volume of at least about 7.5 gms/gm immediately after creping.
Creping methods useful in connection with the present invention include creping with conventional (beveled or unbeveled) crepe blades or, more preferably in some cases with an undulatory creping blade operative to impart a biaxially undulatory structure to the product. The web is generally creped from the heated cylinder with a creping blade defining a pocket angle of from about 50 to about 100 degrees, which blade (as noted above) may be a beveled creping blade. A pocket angle from about 65 to about 90 degrees may be preferred with a beveled blade have a bevel of from about 8 to about 12 degrees, or a bevel of from about 14 to about 18 degrees. When an undulatory creping blade is used, it is typically configured so as to form a reticulated biaxially undulatory product with crepe bars extending in the cross direction and ridges extending in the machine direction. The product of an undulatory wet crepe operation generally has from about 10 to about 150 crepe bars per inch, and from about 10 to about 50 ridges per inch extending in the machine direction.
The method of the present invention is suitably practiced wherein the aqueous furnish includes recycled fiber. The recycled fiber in the aqueous furnish may comprise at least about 50 percent by weight of the fiber present or may comprise at least about 75 percent by weight of the fiber present. In some embodiments, the cellulosic fiber present in the aqueous furnish consists entirely of recycled fiber.
The heated gaseous medium is typically heated by way of combustion or infra-red (xe2x80x9cIRxe2x80x9d) radiation or combinations thereof.
The web may be wet-shaped subsequent to creping and prior to being dried with impinging air by way of vacuum-molding in an impression fabric.
In another aspect of the present invention, there is provided a method of making absorbent sheet from cellulosic fiber comprising: (a) depositing an aqueous cellulosic furnish on a foraminous support; (b) compactively dewatering the furnish to form a web; (c) applying the web to a heated rotating cylinder; (d) maintaining the surface of the rotating cylinder at an elevated temperature relative to its surroundings so as to produce a moisture gradient over the thickness of the web; (e) drying the web on the cylinder to a consistency of between about 40 and about 80 percent; (f) creping the web from said cylinder, wherein the creping is operative to delaminate the web and optionally wet-shaping the web wherein said web is suitable for impingement-air drying; and (g) drying the web with an impinging gaseous medium to form a finished product. The surface of the heated cylinder is generally maintained at a temperature of from about 150xc2x0 F. to about 300xc2x0 F., while the side of the web adjacent to heated cylinder is between about 180 degrees F. and 230 degrees F. upon creping. In some embodiments, steam is supplied to the heated cylinder at a pressure of from about 50 to about 150 psig, whereas steam is preferably supplied to the heated cylinder at a pressure of at least about 100 psig in many embodiments.
The step of drying the web with the impinging drying medium may comprise passing the web through at least one single-wire draw dryer group after creping, each of the at least one single-wire dryer group including first and second rows of web-supporting members and a single drying wire for carrying the web alternating between one of the members in the first row and one of the members in the second row; arranging an impingement-drying device in opposed relationship to at least one of the members in the at least one single-wire draw dryer group; and directing heated air from the impingement-drying device toward the web as the web runs over the at least one member. Typically, members in the first row in a first one of the at least one single-wire draw dryer group in a running direction of the web constitute reversing cylinders and the members in the second row in said first single-wire draw dryer group constitute drying cylinders. The process may further include: arranging the second row of drying cylinders below said first row of reversing cylinders such that said first single-wire draw dryer group constitutes an inverted single-wire draw dryer group; the impingement-drying device being arranged in opposed relationship to at least one of the reversing cylinders in the first row of the inverted single-wire draw dryer group; guiding the web after creping initially over a first one of the drying cylinders in the second row of the inverted single-wire draw dryer group; and passing the web from the inverted single-wire draw dryer group into a twin-wire draw dryer group including first and second rows of drying cylinders, a first drying wire for carrying the web over the first row of drying cylinders and a second drying wire for carrying the web over the second row of drying cylinders. In some embodiments, impinging air is heated to a temperature of from about 150xc2x0 F. to about 300xc2x0 F.
In still further aspects of the invention, there are provided absorbent sheet products made by way of the inventive processes.